For example, in a semiconductor memory device such as a NAND flash memory, a voltage higher than a power supply voltage is used for a data write operation, a data erase operation and a data read operation. Thus, a power supply circuit for generating a voltage higher than the power supply voltage is disposed.
In the meantime, in a boost operation by the power supply circuit, the output voltage is not always a fixed voltage, and oscillates in the neighborhood of a set voltage. This phenomenon is called “ripple”.
In the operation of the memory cell of the NAND flash memory, the ripple of the word line varies the voltage of the word line, causing such an influence as to spread the distribution of a threshold voltage Vth.